Field of the Invention
The invention relates to a component which is suitable for high-frequency applications.
High-frequency application technology makes use, on the one hand, of grounded signals, which are usually referred to in the pertinent technical literature as single ended, and on the other hand of differential signals, which are usually referred to in the pertinent technical literature as balanced signals, are used. Grounded signals are sensitive to fluctuations of the ground potential. By contrast, differential signals, which are preferably symmetrical to the ground potential, are insensitive to shifts of the ground potential.
In certain high-frequency applications, for example between an antenna and a first amplifier stage of a mobile communication device, grounded signals are used, subsequently filtered and converted into differential signals. Impedance matching is often additionally required.
Resonators which have a piezoelectric layer arranged between two electrodes are often used as filters for high-frequency signals (see for example K. M. Lakin et al., IEEE MTT-S International 1995, pages 883-86; K. M. Lakin et al., IEEE Trans. Microwave Theory Techn., Vol. 41, No. 12, 1993, pages 2139-46; and W. W. Lau et al., 1996 IEEE Int. Frequency Control Symp., pages 558-62). If the high-frequency signal is applied to the two electrodes, a mechanical oscillation is induced in the piezoelectric layer. For a wavelength which corresponds to twice the thickness of the piezoelectric layer, a standing wave forms. The frequency selectivity of the impedance of such a resonator can be used for the filtering of signals.
Different types of such resonators have been proposed (see K. M. Lakin et al., IEEE MTT-S International 1995, pages 883-86). On the one hand, the resonator is fastened on a support only at the edge, while the active resonator part is arranged on the surface of a membrane above a cavity. It has also been proposed to isolate the active resonator part by an air gap from a substrate, to which the resonator is fastened at the edge. Finally, it has been proposed to arrange the resonator above a series of layers which reflects the wavelength amplified by the resonator. In that case, the resonator is connected to the substrate over its full width. The series of layers has a plurality of xcex/4 layers and acts as an acoustic mirror. The space requirement of these resonators is typically 200 xcexcmxc3x97200 xcexcm.
These resonators have two electrodes, that is to say they are one-port assemblies, often referred to in the pertinent technical literature as one-port devices.
A two-port assembly is disclosed in K. M. Lakin et al., IEEE Trans. Microwave Theory Techn., Vol. 41, No. 12, 1993, pages 2139-46, with a piezoelectric layer arranged between two electrodes on each of both sides of a substrate. A further two-port assembly of a resonator has two filters stacked one above the other on a substrate. For this purpose, on the substrate there is provided a floating electrode, on that a first piezoelectric layer, on that a grounded electrode, on that a second piezoelectric layer and on that two electrodes arranged alongside each other. That arrangement has a high insertion loss, although stacked filters as a one-port assembly have a high stop-band attenuation. Owing to the two piezoelectric layers required in the two-port assemblies, they entail increased expenditure in comparison with the one-port assemblies.
Another possibility for filtering high-frequency signals is the use of surface-acoustic-wave filters, wherein two electrodes are arranged on the surface of a substrate of piezoelectric material (quartz, lithium niobate). These electrodes are used to induce surface acoustic waves, which propagate in the surface of the substrate. Customary dimensions of surface-acoustic-wave filters are 2xc3x973 mm2.
It is accordingly an object of the invention to provide a component, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which is suitable for high-frequency applications and which can be implemented as a multi-port assembly with a small surface area requirement and low expenditure.
With the foregoing and other objects in view there is provided, in accordance with the invention, a component for forming vertically standing waves of a wavelength xcex, comprising:
a piezoelectric layer having a first main surface, a second main surface opposite the first main surface, and a thickness of substantially xcex/2 or an integral multiple of xcex/2;
a first pair of electrodes including a first lower electrode and a first upper electrode, and a second pair of electrodes including a second lower electrode and a second upper electrode;
the first upper electrode and the second upper electrode being formed on the first main surface of the piezoelectric layer, and the first lower electrode and the second lower electrode being formed on the second main surface of the piezoelectric layer;
the first upper electrode, the second upper electrode, the first lower electrode, and the second lower electrode each being structured such that structures of the first upper electrode and of the second upper electrode engage in one another and structures of the first lower electrode and of the second lower electrode engage in one another.
In other words, the component has a piezoelectric layer with a first pair of electrodes and a second pair of electrodes. The first pair of electrodes has a first lower electrode and a first upper electrode. The second pair of electrodes has a second lower electrode and a second upper electrode. The first upper electrode and the second upper electrode are arranged on a first main area of the piezoelectric layer. The first lower electrode and the second lower electrode are arranged on a second main area of the piezoelectric layer, which lies opposite the first main area. The first upper electrode, the second upper electrode, the first lower electrode and the second lower electrode are in each case structured, the structures of the first upper electrode and the structures of the second upper electrode, and the structures of the first lower electrode and the structures of the second lower electrode respectively engaging in one another. In other words, structures of the first upper electrode and of the second upper electrode and of the first lower electrode and of the second lower electrode are respectively arranged alongside one another, preferably in an alternating manner.
If, for example, a high-frequency signal is applied to the first pair of electrodes, this causes a mechanical oscillation of the piezoelectric layer. The mechanical oscillation forms in the entire piezoelectric layer, including in the regions wherein the surface of the piezoelectric layer is not occupied by the respective electrode between neighboring structures of the first upper electrode or the first lower electrode. What is known as the inverse piezo effect causes this mechanical oscillation between the second lower electrode and the second upper electrode of the second pair of electrodes to generate a high-frequency signal.
The structures of the first upper electrode, of the second upper electrode, of the first lower electrode and of the second lower electrode are preferably so fine in one dimension that they have dimensions which are less than the wavelength of a mechanical oscillation which forms between the first upper electrode and the second upper electrode on the one hand and the first lower electrode and the second lower electrode on the other hand when a high-frequency signal is applied to the first pair of electrodes.
In accordance with an added feature of the invention, the structures of the first upper electrode, of the second upper electrode, of the first lower electrode and of the second lower electrode are so fine in one dimension that they have dimensions which are less than twice the spacing between the upper electrodes and the lower electrodes. This means that these dimensions are less than the wavelength of a high-frequency signal for which a standing wave forms in the layer stack and for which the resonance condition is consequently satisfied.
If the structures of the first pair of electrodes are finer than the resonant wavelength, the influence of these structures on the mechanical oscillation which forms in the entire piezoelectric layer when a high-frequency signal is applied to the first pair of electrodes is reduced. The piezoelectric layer does not xe2x80x9cseexe2x80x9d these structures on account of the dimensions. What is known as the inverse piezo effect causes this mechanical oscillation between the second lower electrode and the second upper electrode of the second pair of electrodes to generate a high-frequency signal.
The high-frequency signal applied to the first pair of electrodes and the high-frequency signal induced at the second pair of electrodes are electrically isolated from one another. The first lower electrode and the first upper electrode and also the second lower electrode and the second upper electrode can consequently be wired independently of one another. In particular, the component allows the conversion of a grounded signal into a differential signal. For this purpose, the first lower electrode or the first upper electrode of the first pair of electrodes is connected to ground potential, while the other electrode of the first pair of electrodes has the high-frequency signal applied to it. The second upper electrode and the second lower electrode of the second pair of electrodes on the other hand are connected symmetrically to the ground potential. In this way, the grounded input signal is converted into a differential signal at the second pair of electrodes.
The component consequently brings about a filtering of the input signal and a transformation of the input signal simultaneously. In addition, an impedance matching between different impedance levels can be performed in a system over the surface occupied by the first upper electrode and the first lower electrode or the second upper electrode and the second lower electrode. With regard to the impedance matching, it is therefore advantageous to provide the first pair of electrodes and the second pair of electrodes with areas of different sizes parallel to the piezoelectric layer. Consequently both a filtering function and also a transformation and impedance matching can be simultaneously realized when the component is used. This means a saving in space and expenditure in comparison with the known solutions, wherein a number of components, possibly complex switching networks, are required for the filtering, impedance matching and transformation.
The dimensions of the structures in one dimension are preferably less by at least a factor of 3 than twice the thickness of the dielectric layer. The smaller the dimensions of the structures are, the less the structures influence the mechanical oscillations forming or the high-frequency signal induced.
The piezoelectric layer with the first pair of electrodes and the second pair of electrodes may not only be fastened on the surface of a membrane which spans a cavity in a supporting frame but also be isolated by means of an air gap from a substrate to which it is connected and also be realized above acoustic mirror layers, which prevent propagation of the mechanical oscillation into a substrate arranged thereunder.
The thickness of the piezoelectric layer is dimensioned such that a standing wave with a wavelength xcex forms in the piezoelectric layer and the neighboring electrodes or in the piezoelectric layer, one neighboring layer, for example a mirror layer, and the electrodes during the operation of the component. The thickness of the piezoelectric layer is generally xcex/2 or integral multiples thereof. If the standing wave forms in the piezoelectric layer and one neighboring layer, the thickness of the piezoelectric layer is xcex/4 or uneven multiples thereof.
The piezoelectric layer preferably has at least one of the materials AlN, ZnO, PdZrTiOx (PZT) or any other thin-film piezo material. In this case, the thickness of the piezoelectric layer is between 1 xcexcm and 3 xcexcm. The component is consequently suitable as a filter for a frequency range of between 400 MHz and 4 GHz. In this case, the electrodes have minimum dimensions of between 5 xcexcm and 1 xcexcm.
According to one refinement of the invention, the first upper electrode, the first lower electrode, the second upper electrode and the second lower electrode in each case have a web, adjacent to which are substructures, which are connected to the web. The first upper electrode and the second upper electrode and also the first lower electrode and the second lower electrode are arranged in relation to one another in such a way that the substructures of the first upper electrode and the substructures of the second upper electrode are arranged in an alternating manner and the substructures of the first lower electrode and the substructures of the second lower electrode are arranged in an alternating manner.
The substructures may, in particular, be designed in the form of fingers, so that the respective electrode has a comb-shaped form and the first upper electrode and the second lower electrode are arranged interdigitally.
Alternatively, they may be designed in the form of branches or trees. In this case, the substructures of the upper electrode can be distinguished from the substructures of the lower electrode, with overlapping regions existing between the substructures of the upper electrode and the substructures of the lower electrode.
For the effective suppression of lateral modes during the inducement of the mechanical oscillation, it is advantageous to make the dimensions and spacings between the substructures in the component different, the dimensions and spacings varying in each case about a mean value. In particular, it is possible to vary the form of the substructures in such a way that their contour has no right angle.
The webs of the first upper electrode and of the second upper electrode may be aligned parallel to the webs of the first lower electrode and of the second lower electrode. Alternatively, the webs of the upper electrodes are rotated with respect to the webs of the lower electrodes. This has the advantage that the influence of the structuring of the individual electrodes on the mechanical oscillations forming is further reduced.
In addition, the upper electrodes may be structured differently from the lower electrodes. This difference may relate both to the form of the structuring and to the dimensions of the structuring.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a component that is suitable for high-frequency applications, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.